JDK源碼解析之 java.lang.System

一個和系統環境進行交互的類.
System不允許被實例化, 而且是一個final類

一、不能實例化

private System() {
}

二、成員變量

public final static InputStream in = null;	//這是“標準”輸入流。
public final static PrintStream out = null;	//這是“標準”輸出流。
public final static PrintStream err = null;	//這是“標準”錯誤輸出流。private static native void setIn0(InputStream in);private static native void setOut0(PrintStream out);private static native void setErr0(PrintStream err);

定義的三個IO流，都是final修飾符，所以即使是public 也是不能重新賦值。

三、常用方法

1、關于SecurityManager

/*** The security manager for the system.*/private static volatile SecurityManager security = null;private static void checkIO() {SecurityManager sm = getSecurityManager();if (sm != null) {sm.checkPermission(new RuntimePermission("setIO"));}}public static void setSecurityManager(final SecurityManager s) {try {s.checkPackageAccess("java.lang");} catch (Exception e) {// no-op}setSecurityManager0(s);}private static synchronized void setSecurityManager0(final SecurityManager s) {SecurityManager sm = getSecurityManager();if (sm != null) {// ask the currently installed security manager if we// can replace it.sm.checkPermission(new RuntimePermission("setSecurityManager"));}if ((s != null) && (s.getClass().getClassLoader() != null)) {// New security manager class is not on bootstrap classpath.// Cause policy to get initialized before we install the new// security manager, in order to prevent infinite loops when// trying to initialize the policy (which usually involves// accessing some security and/or system properties, which in turn// calls the installed security manager's checkPermission method// which will loop infinitely if there is a non-system class// (in this case: the new security manager class) on the stack).AccessController.doPrivileged(new PrivilegedAction<Object>() {public Object run() {s.getClass().getProtectionDomain().implies(SecurityConstants.ALL_PERMISSION);return null;}});}security = s;}/*** Gets the system security interface.** @return if a security manager has already been established for the current*         application, then that security manager is returned; otherwise,*         <code>null</code> is returned.* @see #setSecurityManager*/public static SecurityManager getSecurityManager() {return security;}

System類定義了安全管理器，并且【volatile】修飾該變量。在初始化IO流時，需要使用安全器校驗權限。

2、關于console

控制臺定義console

private static volatile Console cons = null;/*** Returns the unique {@link java.io.Console Console} object associated* with the current Java virtual machine, if any.** @return  The system console, if any, otherwise <tt>null</tt>.** @since   1.6*/public static Console console() {if (cons == null) {synchronized (System.class) {cons = sun.misc.SharedSecrets.getJavaIOAccess().console();}}return cons;}

System中的console是通過【sun.misc.SharedSecrets】類獲取得到的。關于SharedSecrets類這里只能簡單說是關于jvm的

3、currentTimeMillis和nanoTime

獲取系統時間方法

/*** 獲取毫秒級的時間戳(1970年1月1日0時起的毫秒數)*/
public static native long currentTimeMillis();
/*** 獲取納秒，返回的可能是任意時間（主要用于衡量時間段）*/
public static native long nanoTime();

4、arraycopy方法

該復制為淺復制，即對象數組，只復制對象引用。

public static native void arraycopy(Object src,  int  srcPos,Object dest, int destPos,int length);

5、identityHashCode 方法

返回對象地址方法

public static native int identityHashCode(Object x);

6、加載動態庫library

@CallerSensitivepublic static void load(String filename) {Runtime.getRuntime().load0(Reflection.getCallerClass(), filename);}/***/@CallerSensitivepublic static void loadLibrary(String libname) {Runtime.getRuntime().loadLibrary0(Reflection.getCallerClass(), libname);}/*** Maps a library name into a platform-specific string representing* a native library.** @param      libname the name of the library.* @return     a platform-dependent native library name.* @exception  NullPointerException if <code>libname</code> is*             <code>null</code>* @see        java.lang.System#loadLibrary(java.lang.String)* @see        java.lang.ClassLoader#findLibrary(java.lang.String)* @since      1.2*/public static native String mapLibraryName(String libname);

7、初始化Java class

/*** Create PrintStream for stdout/err based on encoding.*/private static PrintStream newPrintStream(FileOutputStream fos, String enc) {if (enc != null) {try {return new PrintStream(new BufferedOutputStream(fos, 128), true, enc);} catch (UnsupportedEncodingException uee) {}}return new PrintStream(new BufferedOutputStream(fos, 128), true);}/*** Initialize the system class.  Called after thread initialization.*/private static void initializeSystemClass() {// VM might invoke JNU_NewStringPlatform() to set those encoding// sensitive properties (user.home, user.name, boot.class.path, etc.)// during "props" initialization, in which it may need access, via// System.getProperty(), to the related system encoding property that// have been initialized (put into "props") at early stage of the// initialization. So make sure the "props" is available at the// very beginning of the initialization and all system properties to// be put into it directly.props = new Properties();initProperties(props);  // initialized by the VM// There are certain system configurations that may be controlled by// VM options such as the maximum amount of direct memory and// Integer cache size used to support the object identity semantics// of autoboxing.  Typically, the library will obtain these values// from the properties set by the VM.  If the properties are for// internal implementation use only, these properties should be// removed from the system properties.//// See java.lang.Integer.IntegerCache and the// sun.misc.VM.saveAndRemoveProperties method for example.//// Save a private copy of the system properties object that// can only be accessed by the internal implementation.  Remove// certain system properties that are not intended for public access.sun.misc.VM.saveAndRemoveProperties(props);lineSeparator = props.getProperty("line.separator");sun.misc.Version.init();FileInputStream fdIn = new FileInputStream(FileDescriptor.in);FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);FileOutputStream fdErr = new FileOutputStream(FileDescriptor.err);setIn0(new BufferedInputStream(fdIn));setOut0(newPrintStream(fdOut, props.getProperty("sun.stdout.encoding")));setErr0(newPrintStream(fdErr, props.getProperty("sun.stderr.encoding")));// Load the zip library now in order to keep java.util.zip.ZipFile// from trying to use itself to load this library later.loadLibrary("zip");// Setup Java signal handlers for HUP, TERM, and INT (where available).Terminator.setup();// Initialize any miscellenous operating system settings that need to be// set for the class libraries. Currently this is no-op everywhere except// for Windows where the process-wide error mode is set before the java.io// classes are used.sun.misc.VM.initializeOSEnvironment();// The main thread is not added to its thread group in the same// way as other threads; we must do it ourselves here.Thread current = Thread.currentThread();current.getThreadGroup().add(current);// register shared secretssetJavaLangAccess();sun.misc.VM.booted();}private static void setJavaLangAccess() {// Allow privileged classes outside of java.langsun.misc.SharedSecrets.setJavaLangAccess(new sun.misc.JavaLangAccess(){public sun.reflect.ConstantPool getConstantPool(Class<?> klass) {return klass.getConstantPool();}public boolean casAnnotationType(Class<?> klass, AnnotationType oldType, AnnotationType newType) {return klass.casAnnotationType(oldType, newType);}public AnnotationType getAnnotationType(Class<?> klass) {return klass.getAnnotationType();}public Map<Class<? extends Annotation>, Annotation> getDeclaredAnnotationMap(Class<?> klass) {return klass.getDeclaredAnnotationMap();}public byte[] getRawClassAnnotations(Class<?> klass) {return klass.getRawAnnotations();}public byte[] getRawClassTypeAnnotations(Class<?> klass) {return klass.getRawTypeAnnotations();}public byte[] getRawExecutableTypeAnnotations(Executable executable) {return Class.getExecutableTypeAnnotationBytes(executable);}public <E extends Enum<E>>E[] getEnumConstantsShared(Class<E> klass) {return klass.getEnumConstantsShared();}public void blockedOn(Thread t, Interruptible b) {t.blockedOn(b);}public void registerShutdownHook(int slot, boolean registerShutdownInProgress, Runnable hook) {Shutdown.add(slot, registerShutdownInProgress, hook);}public int getStackTraceDepth(Throwable t) {return t.getStackTraceDepth();}public StackTraceElement getStackTraceElement(Throwable t, int i) {return t.getStackTraceElement(i);}public String newStringUnsafe(char[] chars) {return new String(chars, true);}public Thread newThreadWithAcc(Runnable target, AccessControlContext acc) {return new Thread(target, acc);}public void invokeFinalize(Object o) throws Throwable {o.finalize();}});}